1. Field of the Invention
This invention relates to a control apparatus for an industrial robot. More particularly, the present invention relates to a control apparatus which is advantageously used for controlling an articulated industrial robot of the type which holds a workpiece for movement relative to a fixed tool.
2. Description of the Prior Art
Industrial robots are widely used in various applications for greatly reducing the burden of workers in factories. The robot is also used for performing a job with extreme precision and/or in a dangerous site.
Conventionally, an articulated industrial robot is made to hold a tool such as a grinder, a welding torch or a cutter for movement relative to a workpiece removably held by a fixed workpiece holder. In this case, therefore, it is necessary to provide a separate transfer mechanism for transferring the workpiece to and from the workpiece holder. As a result, the overall arrangement tends to become bulky and complicated.
In view of the above problem, it has been proposed to cause an articulated industrial robot to hold a workpiece for movement relative to a fixed tool, as disclosed for example in Japanese Patent Applications Laid-open Nos. 64(1989)-37603, 2(1990)-82302 and 3(1991)-239486. According to this proposal, the robot can be also utilized for transferring the workpiece to and from the tool within the maximum movable space of the robot, thereby streamlining the process without requiring a separate transfer mechanism.
According to the teaching of Japanese Patent Applications Laid-open Nos. 2(1990)-82302 and 3(1991)-239486 mentioned above, the control apparatus for the robot prepares a plurality of teaching data (constituting a task program) by memorizing, at each teaching point, the positional and attitudinal relation of a robot mechanical interface point (tip end) relative to a robot base reference point or the positional and attitudinal relation of the robot base reference point relative to the mechanical interface point. In other words, the control apparatus memorizes only the desired robot movement as the teaching data.
According to the teaching of Japanese Patent Applications Laid-open Nos. 64(1989)-37603, on the other hand, the robot control apparatus prepares a plurality of teaching data by memorizing the target positions of the workpiece relative to the robot mechanical interface point.
Further, the prior art control apparatus disclosed in the three laid-open Japanese applications equally controls the robot on the premise that the tool and the workpiece configuration will not be changed.
The prior art control apparatus described above has been found to have various disadvantages. Several of these disadvantages are briefly enumerated below. The reasons for the disadvantages of the prior art will be readily understood from the detailed description of the present invention given hereinafter and therefore not specifically described here. (1) The task program including the teaching data as to the relation between the robot tip end and the robot base or the robot tip end and the workpiece is not directly indicative of the relation between the tool and the workpiece, so that it is difficult to realize, from the task program, how the workpiece is worked on by the tool.
(2) It is difficult to use the CAD (computer aided design) data of a workpiece for making a task program.
(3) It is difficult to modify the task program for adaption to a change in the position and/or attitude of the tool, to a change in the configuration and/or dimension of the workpiece, and to a change in the workpiece holding position provided by the tip end of robot.
(4) The robot is capable of operating only with respect to a single tool. Thus, it is impossible for the single robot to perform successive jobs on the workpiece by a series of tools.